Also why is America the only country that uses a farenheit scale? It's so confusing!
I realize the question is rhetorical, but, basically, because the push for metrification in the 1970s was voluntary, and people refused to change.
We'd still probably be using SAE nuts and bolts were it not for the fact it would have destroyed the ability to market things like cars, trucks, and even military hardware internationally.
Another case where lack of government imposing something has come back to bite us in the posterior (and on multiple occasions). We should have gone to the metric system of weights and measures long ago, but haven't. As a result, I still don't "get" celsius temperatures to this day, which Farenheit still feels "natural," as do old distance measures like miles. It would be far harder for me to mentally change over now, but I'd do it in a minute were it to become the law of the land.